1. Field of the Invention
One embodiment of the invention disclosed herein relates to a power transmission device and a power feeding system.
2. Description of the Related Art
Various electronic appliances have spread, and a variety of products is shipped to the market. In recent years, portable electronic devices such as cellular phones and digital video cameras have widely spread. Further, electric propulsion moving vehicles that are powered by electric power, such as electric cars, appear on the market as products.
In such mobile phones, digital video cameras or electric propulsion vehicles, power storage devices serving as power storage means (referred to as batteries and storage batteries) are incorporated. Currently, power feeding to such a power storage device is performed in most cases by direct connection to a household AC power source which is a power feeding means. In addition, a structure without a power storage device or a structure which does not use electric power fed to a power storage device is directly fed with power from a household AC power source through a wiring or the like so that a unit operates.
On the other hand, methods by which electric power is fed to power storage devices without making contact or electric power is fed to loads without making contact have been researched and developed. Typical methods are an electromagnetic coupling method (also referred to as electromagnetic induction method, see Patent Document 1), a radio wave method (also referred to as microwave method), and a magnetic resonant method (also referred to as resonant method, see Patent Documents 2 and 3).
As described in Patent Document 2, in a contactless power feeding technique using a magnetic resonant method, a device that receives electric power (hereinafter, referred to as power receiving device) and a device that supplies power (hereinafter, referred to as power transmission device) each have a resonant coil. Further, in each of the power receiving device and the power transmission device, an electromagnetic induction coil is provided. Feeding of power from a power source to the resonant coil in the power transmission device and feeding of power from the resonant coil to a load in the power receiving device are conducted by the electromagnetic induction coils.
In the resonant coil of the power transmission device and the resonant coil of the power receiving device, resonance frequencies (LC resonance) are adjusted so that a magnetic resonance phenomenon occurs at a particular frequency.
When the resonant coil of the power transmission device and the resonant coil of the power receiving device face each other, they makes a magnetic resonance phenomenon, and thereby efficient power transfer is realized even when the distance between the resonant coils is large (see Non-Patent Document 1).
The above contactless power feeding technique is favorable because wireless communication between the power transmission device and the power receiving device and safety management such as individual identification and detection of foreign substances can be performed at the same time as contactless power feeding. In addition, the above contactless power feeding is favorable because during contactless power feeding, communication of information about power and charge of the power receiving device can be performed and the power transmission device can perform control of power feeding, such as adjustment of power.
To perform wireless communication between the power transmission device and the power receiving device at the same time as contactless power feeding, both an antenna for communication and an antenna for power feeding are prepared. However, as disclosed in Patent Document 4, when one antenna can be used for both communication and power feeding, it is possible to reduce the size, weight, and cost of a power feeding system including the power transmission device and the power receiving device.